In recent years, multi-antenna systems have shifted from a theoretical research stage to large-scale applications in modern wireless cellular systems. In today's wireless communication field, whether in the actual system or in the theoretical research, inter-cell interference has become a bottleneck, and the bottleneck seriously limits channel capacity and throughput of communication systems. In order to reduce the inter-cell interference, the current research focus is a Multiple-Input Multiple-Output (MIMO) interference alignment technology, and a coordinated network. It has been proved that the technologies can effectively reduce the inter-cell interference. However, facing the amount of data increasing exponentially, comprising wireless phone businesses, as well as constantly increasing demands on wireless data, the technologies neither can fundamentally advance the system capacity by leaps and bounds, nor can meet users' needs.
Massive MIMO is an emerging technology that equips the base stations (BSs) with a large number of antennas to improve spatial resolution and antenna array gain, it can provide a higher data rate with lower power consumption, to cause spectrum utilization to reach an unprecedented level, and thus becomes a potential candidate technology for the-fifth generation cellular networks.
A main problem for Massive MIMO is the availability of instantaneous Channel State Information (CSI). In such a wireless transmission scheme, the base station side has a huge number of low-power small antennas, the number of the antennas is much greater than the number of single-antenna users scheduled at the same time. Massive MIMO can make a wireless communication system achieve a high throughput, and when the number of the antennas of the base station tends to be infinite, channel capacity should be infinite. However, in a practical application scenario, this does not work. The only limiting factor is Pilot Contamination. In a Time-Division Duplex (TDD) system with ideal channel reciprocity, the CSI can be acquired through uplink pilot-aided training. Due to coherence interval with a limited length, the number of available orthogonal pilot sequences is limited. Therefore, the pilot sequences will be reused among several cells, users in adjacent cells sending the same pilot sequence will result in that a result of the channel estimation may not be of the channel between a desired user and the base station, but the estimation contaminated by training sequences sent by users in other cells. As the number of antennas goes to infinite, the Pilot Contamination becomes the bottleneck of the system performance (downlink transmission performance).